The underlying premise of this proposal is that abnormal in vitro CTL generation mirrors abnormal in vivo T cell-mediated immunity in SLE and has important implications for pathogenesis and/or development of the immunocompromised state seen in SLE. In SLE PBMC cultures, generation of unrestricted cytolytic activity in response to anti-CD3 mAb is usually abnormal despite normal corresponding proliferative responses, with two predominant abnormal anti-CD3-induced cytolytic response patterns existing among SLE patients, suggesting two predominant discrete mechanisms underlying the dichotomy between normal anti-CD3-induced proliferation vs abnormal anti-CD3-induced unrestricted cytolytic activity in SLE. To assess the biologic importance and cellular mechanisms of abnormal generation of CTL in SLE, we will address two major questions. 1) Is abnormal generation of CTL activity inherent to the SLE patient, or is abnormal CTL activity secondary to the disease? We will study individual SLE patients over time and correlate changes in anti-CD3-induced cytolytic lytic with changes in clinical parameters. The existence of such correlations would suggest that factors associated with the SLE disease were causing the impaired cytolytic response rather than abnormal CTL generation being primary. To document that the impaired CTL activity in SLE is not simply a reflection of in vivo "immune activation", patients with non-SLE immune-based rheumatic diseases will also be longitudinally studied. In parallel studies, monozygotic twins discordant for SLE will be evaluated. Should the healthy co-twin exhibit the same defect in CTL generation as does the SLE co-twin, it would suggest that the defect antedates the onset of SLE and may be a predisposing factor. 2) What are the mechanisms underlying abnormal generation of CTL activity in SLE? Five broad mechanistic possibilities will be considered. a) Is there a defect extrinsic to the T cell resulting in abnormal CTL generation? the contribution of monocytes and serum factors to abnormal CTL generation in SLE will be assessed. b) Is there a maturational defect in SLE T cells? Phenotypic markers of maturation and the effects of decreased phenotypic maturation on attachment of CTL to their targets will be monitored. Should global T cell maturational defects be found, global abnormalities in generation of intracellular second messengers will be sought. c) Is there a differentiation defect in SLE T cells? Cytokine production patterns will be determined to assess a shift in SLE from Th1-like cell differentiation to Th2-like cell differentiation (leading to increased T cell helper activity with decreased T cell cytolytic activity). Should such a shift be found, exogenous cytokines will be added to the cultures in attempt to overcome the abnormal differentiation. d) Is there a defect limited to discrete T cell subsets? We will monitor those numerically small T cell subsets known to disproportionately contribute to cytolytic activity for selective defects. e) Is there a defect in cross-talk among T cell subsets? T cell subsets will be tested individually and in defined combinations to assess the possibility of there being excessive inhibitory cross-talk in SLE.